1. Field of the Invention
The present invention relates to methods for controlling the evolution of stress during an electroplating process.
2. Description of Related Art
In an electroplating process, a particular phenomenon occurs in that all electroplated metals tend to shrink or expand relative to their substrate during or after the plating process. Electroplated metals that are under tensile or compressive stresses may: peel and crack, and create non-uniform plated sections causing dimensional instability of electroformed sections and increase vulnerability to corrosive attack. Thus, in general, stress in electroplating is undesirable.
Stress is of especially a great concern in micro-electro-mechanical systems such as micro sensors and microelectronics. Examples of micro sensors are accelerometers and glyoscopes which, are used in applications including but not limited to aerospace and automotive. Due to the high precision required in these systems, any stress at the electroplated metal will have a pronounced effect.
In 1958, Joseph B. Kushner, a professor of Engineering at Evansvile College, Indiana, conducted research of the principal factors affecting plating stresses including plating temperature, film thickness, plating current density, and the influence of contaminants. Related to his research, Joseph B. Kushner published an article entitled Stress in Electroplated Metals in a trade journal called Metal Progress, on Feb. 22, 1962. His research results showed that all electroplated metals shrink or expand relative to their substrate during or after the plating process. This, in fact, is due to tensile or compressive stresses. In his case study of rhodium plating, the tensile stress developed ran as high as 100,000 psi. Experimenting with deposit thicknesses, he found that with the exception of the initial stage of deposition, tensile stress decreases as the deposition thickness increases.
A complete description on the subject of metal stresses is beyond the scope of the specification. For details, and for an extensive bibliography of references on metal stresses, see J. W. Deni, Stress, published in a book entitled Electrodeposition by Noyce Publications of New Jersey in 1993.
A commonly known equation used in the electroplating industry is the Stoney Equation. The Stoney Equation calculates the average stress in an electroplated metal. The equation is as follows: ##EQU1##
where
E is the Young's modules of the substrate, PA1 V is the Poisson's ratio of the substrate, PA1 T, is the thickness of the substrate, PA1 r is the radius of the wafer, PA1 h is the displacement of the wafer at the center, and PA1 T.sub.f is the thickness of the film. PA1 The Need for an Alternative Opaque Layer," Mat. Res. Soc. Symp. Proc., Vol. 356, 1995, pp. 239-244; and A. Brenner and s. Senderoff, "Calculation of Stress in Electrodeposits for the Curvature of a Plated Stip," U.S. Department of Commerce, National Bureau of Standards, Research Paper RP1954, Vol. 42, February 1949. PA1 1. A dummy part and a second setup are being used for measuring and data gathering purposes instead of using the actual part being electroplated. Thus, an actual part that uses a different shape or a different material from the dummy part will cause errors. PA1 2. A strain gauge is needed to be glued onto the substrate being measured. PA1 3. The strain gauge glued onto the substrate will destroy the substrate being measured; PA1 4. The strain gauge has low sensitivity and is inherently imprecise due to its mechanical nature; PA1 5. The cathode on the dummy part and the second setup needs to be replaced after each run. Thus, the material cost is higher. PA1 6. High part content because an additional cathode and an additional power supply is needed for the dummy part and second setup; and PA1 7. High system cost due to high part content.
A positive stress represents the tensile stress while negative stress implies the compressive stress in the electroplated metals. A further explanation of the Stoney equation can be found in the following publications: C. M. A. Ashruf, P. J. French, C. de Boer and P. M. Sarro, "Strain Effects in Multi-Layers," SPIE Vol. 3223, 1997, pp. 149-159; J. A. Cairns, C-H. Liu, A. C. Hourd, R. P. Keatch and B. Lawrenson, "Potential Limitations of Conventional Photomask to Inherent Internal Stress
A method for controlling stress induced by electroplating is known in the prior art, being disclosed in U.S. Pat. No. 4,648,944 to Ronald George, et al. Specifically disclosed is a monitoring system consisting of a strain gauge, a strain gauge monitor, several DC current regulated programmable power supplies, and a computer controlling the power supplies. The method of the prior art has disadvantages, including the following:
Somewhat related to this application is Kubona et al., U.S. Pat. No. 5,666,253, Method of Manufacturing Single Wafer Tunneling Sensor. The patent discloses a method of photo lithographically fabricating a unitary structure sensor on a semiconductor substrate. A cantilever beam is formed on the substrate, while the centilever beam has a nickel plating. It is through the process of electroplating nickel on the cantilever beam that the problem of metal stress was investigated.
Thus, there is a need for a method of in-situ displacement/stress control in electroplating that avoids the disadvantages of the prior art. The specific need is to have a more accurate measurement of the displacement of the substrate instead of the usage of a dummy part. In addition, the need to have a lower system cost by reducing unnecessary or redundant components.